A composite video baseband signal (CVBS), a widely used analog video signal, is generated by mixing a luma signal and a chroma signal of an image. Three classifications of specifications associated with the CVBS are the National Television System Committee (NTSC) specification, the Phase Alternating Line (PAL) specification, and the SECAM specification.
FIG. 1 shows a waveform diagram of the CVBS. A synchronization prompt signal 10 is used for marking a start point of each of horizontal scan lines. A sinusoidal color burst signal 12 provides a reference signal associated with chromaticity. A staircase wave after the color bust signal 12 represents a luma signal of an image signal. For example, in FIG. 1, VMAX may correspond to a luminosity having a grayscale value 255, and VMIN may correspond to a luminosity having a grayscale value 0.
In the SECAM specification, a chroma signal is represented by a sinusoid added to a luma signal, such as the sinusoid signal added to the luma signal illustrated in FIG. 1. A chromaticity (color) is calculated by comparing angular frequencies of the chroma signal and a carrier burst signal.
FIG. 2 is an image processing circuit for decoding chroma components of a SECAM signal of the prior art. An image processing circuit 1 comprises a receiving unit 13, a filter unit 11, a frequency demodulation module 14 and a chroma converting unit 16. The filter unit 11 is coupled to the receiving unit 13, the frequency demodulation module 14 is coupled to the filter unit 11, and the chroma converting unit 16 is coupled to the frequency demodulation module 14.
The receiving unit 13 receives images encoded according to the SECAM specification. An images signal of one certain image among the received images is represented by S (S=Y+Sin [(ωc+Δω)*t+φ1]), where Y is a luma signal, Sin [(ωc+Δω)*t+φ1] represents a chroma signal, ωc is a carrier angular frequency of the SECAM system, Δω is an angular frequency difference between an oscillator angular frequency and a carrier angular frequency of the image signal, and φ1 is a phase. Since the chroma signal based on the SECAM system is encoded via a frequency modulation approach, Δω may correspond to a chromaticity.
The filter unit 11 mainly filters out luma components (the luma signal) from the image signal, and comprises a band-pass filter (not shown) and a cloche filter (not shown). Only chroma components (the chroma signal) remain in the image signal after the image signal is filtered, and the filtered chroma signal is then processed by the frequency demodulation module 14 to calculate Δω. After that, the chroma converting unit 16 generates a chromaticity (color) corresponding to Δω. The processes of the frequency demodulation module 14 and the chroma converting unit 16 are known to a person having ordinary skill in the art, and thus shall not be discussed for brevity.
In the SECAM system, a chroma signal of an image oscillates at a carrier angular frequency, which is 4.25 MHz or 4.41 MHz. When a luma signal illustrated in FIG. 3A oscillates at an angular frequency close to the carrier angular frequency, the luma signal cannot be filtered out via an image processing method of the prior art for the reason that high frequency components of the luma signal is taken into consideration in a frequency demodulation calculation of the chroma signal. Therefore, in the frequency demodulation calculation of the image processing method, regular oscillation of the luma signal corresponds to a chromaticity (color), thus resulting in abnormal color blocks in a grayscale multi-burst pattern. FIG. 3B is a schematic diagram of resulting abnormal color blocks in a multi-burst pattern, in which various black blocks are arranged at an interval and color blocks 20 are formed.
Therefore, a main object of an image processing method and an image processing circuit according to an embodiment of the present invention is to prevent formation of abnormal color blocks in a multi-burst pattern.